Staggered grids: allow more than just face neighbors

When defining a kernel on a staggered grid, one can currently only store values on the faces/edges (3D/2D) of a cell. This is sufficient for common finite volume schemes. However, when one has a specific discretization in mind (in my case, Capuani's electrokinetics solver), one may also need to calculate fluxes for the edges/corners (3D/2D). For a volume-of-fluid-like scheme, one also needs the corners in 3D.

So currently we only support D3Q6/D2Q4 staggered grids and I need D3Q26/D2Q8. This was already discussed with @bauer and is probably not a whole lot of work to implement.